1. Field of the Invention
This invention relates generally to projection lens systems of the type used in connection with cathode ray tubes and specifically relates to such a system having an internal lens folding means.
2. Description of the Prior Art
Heretofore, consumers desiring to enjoy the advantages of large screen projection color television had to choose between the lesser of two evils, figuratively speaking. They could purchase an expensive multiple cathode ray tube projector specifically designed for large screen projection, or they could purchase a lens system adaptable to the single cathode ray tube of their existing set. Selection of either option required making a trade-off. For example, the multiple tube system is limited to applications wherein the projection screen has a luminance gain of 10 or less because two or more colors are projected along different optical axes, and color shifting occurs due to the differential in reflectance angles. Projection from a single cathode ray tube does not suffer from this color shifting limitation, since all colors are projected along a common axis of the cathode ray tube.
The single color cathode ray tube can therefore be used in applications wherein the projection screen luminance gain is much greater than 10. The use of higher gain screens compensates, at least to a limited degree, for the luminous inefficiency of the cathode ray tube. Unfortunately, the limitations of the prior art lenses have prevented the use of single cathode ray tubes with projection screens having a gain higher than 13. Specifically, prior art lenses are restricted to about 13 gain screens to avoid a serious loss of viewing angles, which loss would render such lenses unacceptable to consumers. Prior art lenses are limited in speed to about f/1.9, to control their diameters to thereby permit a limited folding of the image beam, and by optical aberrations. This inability of prior art lens systems to be used with single cathode ray tubes in conjunction with projection screens having a gain of 13 or greater has the effect of wasting the capability of higher gain screens to provide brighter images .
The increased specularity (brightness) of high gain screens also enables them to reject ambient light, and ambient light rejection enhances the contrast ratio range of the projected image. Although CRT tube brightness is important, in the limit, the ability of the projection screen to reject ambient light is even more critical than the brightness of the tube. This follows because while tube brightness influences high-lights, the black range is influenced by a screen's light rejection or room darkening control.
Prior art lenses have focal lengths of 300 mm or more, and generate incidence angles of up to 13 degrees which, when used with a 13 gain screen, provide acceptable viewing at 25 degrees half angles of 15 percent of the on-axis brightness (about 2 gains). It is desirable to achieve such a viewing angle with a 17 gain screen, so that the above-described characteristics of a high gain screen could be harnessed. However, to achieve such a viewing angle with a 17 gain screen would require a vertical lens incidence angle of 17 degrees (a focal length of 280 mm) and a lens speed of f/1.5. However, as we have seen, prior art lenses are restricted to incidence angles of less than 13 degrees, because prior art lens speeds are limited in speed to only f/1.9. Clearly, then, the prior art lenses are prevented from harnessing the desirable characteristics of 17 or higher gain screens as a result of the aforesaid limitations inherent in prior art systems. If these limitations could be overcome, a substantial increase in contrast could be achieved. Accordingly, the image on such a screen would be perceived as being substantially brighter.
A lens of a speed of f/1.44 (as measured on the image side), with a screen of a brightness of 17 gains and a standard 13 inch cathode ray tube will produce an image of superior contrast to most multi-tube projectors using screens of 8 gains. The observer will perceive this superior contrast as a brighter picture, though in fact it is less bright. Contrast will favor the single tube projectors down to 5 foot candles of ambient light; at that point, and darker, the picture of the multi-tube projector will dominate.
Still further limitations appear in prior systems. Image folding techniques associated with prior art lenses place the television behind the mirror and under the screen, making the controls and internal loudspeaker inoperative. The mirror required with prior art systems averages 14 inches.times.17 inches, and is subject to dirt accumulations, mounting distortion, weight and cost penalties. Prior art systems also require that the television face toward the audience, glaring over the top of the mirror-holder into the eyes of the viewer.
Keystone distortion results from the absence of perpendicularity between the screen and image. When the top of the image beam travels farther to the screen, it is enlarged relative to rays traveling a shorter distance, resulting in egg-shaped heads, bowed titles, etc. Keystone distortion is most noticeable in off-axis projection using projectors of considerable luminosity, or projection onto screens of a brightness rating of 5 gains or greater; however, due to the law of reflection, a perpendicular condition would reflect the primary rays back to the projector, so perpendicularity must be avoided to redirect the reflected image back into the audience zone. While prior art lenses have an actual beam angle up to 16 degrees, due to the aspect ratio (the vertical height) of the image source and projection screen, their reflected beam angles are restricted to 13 degrees or less, producing a minimum of 56 percent keystone distortion (the screen should be oriented at 30 degrees from the vertical to be perpendicular, but can at best be 13 degrees, generating a 17 degree error, or 56 percent distortion). While 56 percent distortion is acceptable to many viewers, it is still distracting to others, particularly because such distortion occurs from the center of the image and upwards. However, it has been demonstrated that distortion under 40 percent is not obvious to most viewers. Also, the wider the vertical beam, the less critical is the screen's angle; accordingly, a 17 degree vertical beam angle appears parallel on a 20 degree screen, which results in only 33 percent distortion--well below industry standards (and 41 percent improved over 13 degree beam angles).
It is clear that there is a long standing and heretofore unfulfilled need for a large screen projection lens system that permits superior projection from standard television sets, while allowing the controls and internal speakers of such sets to remain operative. The improved system will have improved brightness and projection angles over prior art systems. The improved system will also require less space than the prior art systems, and will be less expensive.